Lost 6.6kv phase causes damage

[isquaredr]

Can anyone help explaining this one. A customers has a 1MVA 6.6/0.415Kv Dyn11 transformer
with a solidly earthed neutral. The factory which this transformer supplies has a number
of 3-phase motors supplied from it, a problem occurred on the utility incoming supply
which left only 2-phases connected, the 3rd being open circuit, lots of damage occurred
to 1-phase equipment, contactor coils burnt out etc. I thought with this scenario that 1-lv
phase-N would be at full volts with the other two-N at half volts, so why so much damage.
thanks

 

[alehman]

This doesn't seem to make a lot of sense. Are you sure of the transformer winding connection? Perhaps when the phase opened there was a some arcing which produced large transient voltages.

 

[isquaredr]

Thanks for the reply, it has puzzled me, I can confirm that no arcing happened as the failure was within a 6.6kv oil switch and any extended arcing would have resulted in disruptive failure of that switch.
Anyone else shed some light on this one ?

 

[waross]

50% of rated voltage will burn out most contactor coils and many relay coils.
A contactor at rest has a relatively low impedance as a result of the air gap in the magnetic circuit. Once the contactor is energised and sealed in, the impedance increases. No more air gap means more inductive reactance (impedance) and less current.
If a contactor coil is supplied with a voltage less than the pull-in voltage, the current will be excessive and the coil will burn up.
Check the manufacturers specs for the inrush current of your contactors. Although at half voltage the inrush will be half, it is still usually enough current to fry a coil.
Some relays have a low enough X/R ratio
that the resistance is the major component of the impedance. Such relays will withstand undervoltage conditions much better and some are safe at any reduced voltage.
respectfully

 

[isquaredr]

Thanks Waross thats understood, the only part that does not suit your reply is the contactors were already energised (pulled in)/ motors running. The contactor drop off voltage is usually a lot lower than its pickup voltage therefore the contactors should have stayed in and with the air gap at a minimum with impeadance maximum they should not have burnt out.
Could there be anything else ?

 

[Skogsgurra]

Yes, it is true that a perfectly balanced load will produce 50 % voltage in the affected secondaries. But, are the loads really perfectly balanced? If not, you will have any combination of voltage, from 50/50 to the unlikely 0/100 and everything in-between.

Also, it is true that small contactors sometimes, but not always, stay pulled in at 50 %. But they are not always specified to do that. One authority
(

http://www.measurlogic.com/pdf/dpi/pq3.pdf

) Table 1, mentions drop out voltages between 53 and 68 %. So, according to this source, most of your contactors would drop and be exposed to high current because of the air-gap in the magnetic circuit.

That is also in line with my experience.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[TestBeforeTouch]

I think your problem is probably due to harmonics and negative sequence currents caused by the severe voltage imbalance on your system.

See:

http://www.icrepq.com/PONENCIAS/4.279.QUISPE.pdf

and

http://www.ssac.com/catalog/0807S.pdf

 

[Skogsgurra]

Yes, that is evident. Single-phasing motors get hot. And I think the OP did understands that.

The problem is what happened to single phase loads? Those are already single phase and there are no positive, zero or negative components.

Negative components and harmonics do not explain why contactor coils burn out. A 50 % voltage reduction does. What other single-phase equipment were destroyed?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[TestBeforeTouch]

Harmonics will raise the temperature of the coil through skin effect heating.

 

[Skogsgurra]

You serious? Magnet wire for coils seldom (read never) have dimensions that will cause skin effect at those low frequencies.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Marke]

The contactor magnetic poles will typically begin to separate well above 50% voltage and this will result in an increased current through the coil. The increased current will help to keep the poles partially closed and this causes the coils to burn out.
Most manufacturers design for continuous operation at -15%. The contactors may stay closed electrically down to 50% in some cases, but continuous usage down there will damage the coils. I have seen coils fail at -20%.

I would also expect to see increased contact heating where the contactor is loaded due to reduced contact pressure.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[isquaredr]

Thanks guys for your replies, I guess the only way to prevent this from happening again is to fit an under-voltage protection scheme.
One other possible scenario, what if the star point of the feeding transformer was not earthed and a phase-earth fault was on the system, could the neutral be displaced to possibly give over voltage on one or more phases.

 

[waross]

A first fault on an ungropunded system produces a corner grounded system. There will be two phases with increased voltage to ground, but no other effect.
For example, if a 277/480 volt system is run ungrounded, a ground on one phase will raise the voltage to ground on the other two phases to 480 volts.
This will not affect the voltage supplied to any equipment. Equipment and conductors intended for use on 277/480 volt systems are typically insulated for 480 or 600 volts.
There may be some issues with VFDs and DC drives.
Search this site for information if you have an ungrounded system and drive systems.
respectfully